The age at which reparative surgery can be undertaken in infants with congenital heart disease has steadily fallen over the past ten years. Providing impetus to this direction has been the gratifying results achieved with those lesions that must be repaired early such as total anomalous pulmonary venous return and transposition of the great vessels. It is now recongnized that early surgical repair, especially for infants with cyanotic congenital heart disease, provides better long-term cardiac function. The repair of congenital cardiac defects has been greatly facilitated by techniques which result in a bloodless, nonbeating heart. This has been accomplished by the use of deep hypothermic circulatory arrest. Several questions remain regarding the optimal application of this technique to infants. Answers to these require an improved understanding of the energy metabolism of the immature myocardium. Compared to the mature myocardium the immature heart can withstand longer periods of ischemia, during which time there is better preservation of high energy phosphates. This may be due to differences in energy metabolism. The newborn heart has greater glycogen stores and an increased ability to make use of anaerobic metabolism during periods of ischemia. However, most animal species exhibit a maturational development of oxidative phosphorylation for several days following birth with an increase in mitochondrial density and an alteration in the predominant isoform and content of several enzyme systems involved in aerobic energy metabolism. Thus, although the immature myocardium may have greater resistance to anoxic insult, the ability to maintain high energy flux, with the reinstitution of cardiac contraction following a period of ischemic arrest, may be significantly different from that of the adult heart. How these differences impact survival following open hert surgery is not well understood. Many infants survive surgery only to develop severe myocaridal dysfunction several hours following surgery and much of the mortality in open heart surgery occurs in the early postoperative period. The applicants have developed an in vivo model of cardiopulmonary bypass in the immature heart which provides simultaneous measurement of high energy phosphate homeostasis using phosphorus NMR spectroscopy and multiple hemodynamic measurements of cardiac function. We propose to study aspects of high energy phosphate metabolism in the immature myocardium during and after period of cardiopulmonary bypass and hypothermic circulatory arrest. The purpose of this work is to improve the understanding of those factors responsible for preserving myocardial function following cardiopulmonary bypass in the newborn myocardium.